Can we use other bases instead of KOH or NaOH for soap making?

Two factors to take into account: molar mass and cation size. As KOH is heavier than NaOH, more KOH is required to neutralise the same amount of fatty acids. That means, the fatty acids that do not get neutralised will work as plasticisers and make the soap runnier.

Moreover, as K salts are in general more soluble than Na salts, a soap made out of NaOH is easier to stay solid than a corresponding soap made out of KOH.

Only those metal hydroxides are to be used where the salt is soluble in water. If Ba(OH)2 is used or Mg(OH)2 or Ca(OH)2 , the resultant Ba, Ca or Mg salts of stearic acid or palmitic acid, being insoluble in water will result in scum formation. A shear wastage of the soap. So, Na and K ions are best suggested.

Yes, these are comparatively cheap harmless bases. Ca(OH)2 has also been used for washing soaps. Soaps made of some transition metals are used in making solid lubricants.

yes barium hydroxide for example

It is a long tedious process I do not recommend unless you are already an experienced soap maker. You will need to make your own pot ash from hard wood and add water to the ash. Instead of try to explain it to you I’ll post a link.

How to Make Soap from Ashes - Modern Homesteading - MOTHER EARTH NEWS

Yes, but they would be difficult for a hobbyist, except possibly using potash.

There are two issues with making soap from fats: the catalysis of hydrolysis, and the final product. The word “saponification” now means both “base-catalyzed hydrolysis” and its original meaning of “making into soap”.

If you’re doing the hydrolysis by some other means, as is done in large-scale soap manufacture, then you needn’t concern yourself with the catalytic power of the alkali. You just make your fatty acid, and neutralize it with whatever final base you like. Generally, however, they find it most economic to use the caustics for this purpose anyway.

However, if you’re making soap by kettle process, wherein the catalyst also provides the cation for the final salt. You need the catalyst to be strong enough to do alkaline hydrolysis at a useful rate. My understanding is that potash, K2CO3, is able to substitute for caustic potash, KOH, in this role for making potassium soap, but that sodium carbonate can’t do similarly in place of lye for making sodium soap.

But what about making soaps with other cations? Soaps can be used for various purposes, not always as detergents, so not all of them need be water soluble. There are soaps of various cations used as anti-caking agents, mixed in greases as lubricants, and generally for water-repellent coatings, such as the zinc stearate used in some baby powders. But water-soluble soaps used for cleaning too can have different cations such as triethanolammonium or ammonium, using the bases triethanolamine (TEA) and ammonia respectively. Neutrogena soap is triethanolammonium soap so that at maximal ionization it gives a much lower pH solution than sodium or potassium soap does. However, I think they make it by first hydrolyzing the fat and then neutralizing, rather than direct saponification by TEA.

Ammonia, as a Lewis base, could even do saponification anhydrously as a gas, theoretically. I don’t know if that’s been done practically. There wouldn’t be much use for a pure ammonium soap, it being very stinky, although soaps for washing hands of mechanics have been made with some ammonia.

You could in principle use another group I metal hydroxide like LiOH or CsOH, although the resulting “soaps” would be far more expensive than those from NaOH or KOH. However, the corresponding products from group II hydroxides like Mg(OH)2 or Ca(OH)2 are insoluble in water. We call such materials “soap scums” and their formation is a problem in areas that have hard water.

KOH
The definition of a stronger base is that it can give away OH- as easily as possible. K+ because of its larger size can easily do that.